The cylindrical magnetically-enhanced controlled sputtering cathode systems for coating tubular substrates being now generally proposed belong to the so-called "cylindrical magnetron sputtering" systems, such that proposed in EP Publication 0 045 822, comprising a magnet assembly disposed behind the back side of a tubular target opposed to the sputtering face for generating magnetic fields having flux lines which project curvilinearly from said sputtering face and return thereto thus forming arched flux portions thereover. During sputtering of the cathode target by bombardment with ions of the plasma generated by the glow discharge between said cathode target and an anode, the secondary electrons emitted by the cathode are restrained by the magnetic field into a spirally elongated path. This elongated path of electrons is, according to Penning's principle, responsible for an increased ionization probability of the present gas and, in consequence, for enhancing the sputtering rate of the target.
With such a magnetron, the permanent magnets must be rotated or swung relative to the target in order that the erosion is uniform. The minimum diameter of the tubular target for receiving the permanent magnets of the magnetron cannot be reduced under a limiting value which can be situated around 30 mm. Taking the cathode dark space into account, it is consequently not possible to coat tubular substrates having a diameter under around 60 mm.
Instead of permanent magnets, internally installed, the magnetic field can be created by external magnets or coils. In this case, the target can be reduced to a much smaller diameter. Such a sputtering device, known as "post magnetron" is however limited to a case, where the substrate to be coated internally is not ferromagnetic.
The present invention provides a cylindrical magnetically enhanced sputtering cathode in which the magnetic field at the cathode target is no longer generated by a magnetron; thus, the target erosion is uniform and no drive system for rotating the magnets is needed.
According to the electromagnetic theory, it is well known that when an electric current flows along a conductor, circular magnetic induction lines are built up concentrically around the conductor axis. The Biot-Savart's law specifies that the field intensity decreases with the first power of the radial distance from the conductor.
By biasing negatively a target surface coaxially disposed around the conductor axis, immersed in a low pressure gas, a glow discharge is generated. Electrons, emitted from the target surface are accelerated radially by the electric field, but deviated perpendicularly by the magnetic field. The resulting motion is confined in a spiral shaped area centered on the conductor axis, the closer it is to the target surface, the stronger is the magnetic field.
By increasing the supply current in the conductor, a contraction of the plasma around the target surface will result. The cathodic dark space is reduced to a very narrow zone and the discharge phenomenon can be maintained even when the target surface is surrounded closely by a cylindrical wall of the substrate to be coated.